Surface roughness metrology deals with basic terminology of surface,surface roughness indication methods,analysis of surface traces, measurement methods,surface roughness measuring instruments such as Stylus Probe Instrument, Profilometer, Tomlinson Surface Meter ,The Taylor-Hobson Talysurf etc.This is very useful for diploma,degree engineering students of mechanical,production,automobile branch
2. •Repetitive or random deviations from the normal
surface which form the pattern of the surface. Surface
texture include roughness, waviness, lay and flows.
•Every part’s surface is made up of texture and
roughness which varies due to manufacturing
techniques and the part structure itself.
•Primary texture: lr/hr<50 (Roughness)
•Secondary texture: lw/hw>50 (Waviness)
Surface Texture
4. 1. Roughness :
Roughness consists of surface irregularities which result from the
various machining process. These irregularities combine to form
surface texture.
2. Roughness Height :
It is the height of the irregularities with respect to a reference line. It is
measured in millimeters or microns or micro inches. It is also known as
the height of unevenness.
3. Roughness Width :
The roughness width is the distance parallel to the nominal surface
between successive peaks or ridges which constitute the predominate
pattern of the roughness. It is measured in millimeters.
4. Roughness Width Cut Off :
Roughness width cut off is the greatest spacing of respective surface
irregularities to be included in the measurement of the average roughness
height. It should always be greater than the roughness width in order to
obtain the total roughness height rating.
5. 5. Lay :
Lay represents the direction of predominant surface pattern
produced and it reflects the machining operation used to produce
it.
6. Waviness Height:
Waviness height is the peak to valley distance of the surface
profile, measured in millimeters.
7. Arithmetic Average (AA):
If X is the measured value from the profilometer, then the AA
value can be calculated as shown below.
8. Root Mean Square (rms)
The rms value can be calculated as shown below. Its numerical
value is about 11% higher than that of AA.
9. Flaws: Irregularity which occur at one place or widely varying
intervals in surface. It include cracks, scratches etc.
6.
7. INDICATION OF SURFACE TEXTURE
The basic symbol consists of two legs of unequal
length inclined at approximately 60 degrees to the
line representing the considered surface
The symbol must be represented by thin line
If the removal of material by machining is
required, a bar is added to the basic symbol,
If the removal of material is not permitted,
a circle is added to the basic symbol.
When special surface characteristics have to
be indicated, a line is added to the longer arm of
any of the above symbols,
8. Indication of Surface Roughness
The value or values defining the principal criterion of
roughness are added to the symbols
a- surface roughness value
If it is necessary to impose maximum
and minimum limits of the principal
criterion of surface roughness, both values
shall be shown
maximum limit (a1) ;minimum limit
(a2).
Roughness a
obtained by any
production process
Roughness a
obtained by
removal of material
by machining
Roughness a shall
be obtained without
removal of any
material
9. If it is required that the required surface texture be produced by one
particular production method, this method shall be indicated in
plain language on an extension of the longer arm of the symbol
Roughness
Value
(Microns)
Roughness
Grade
Conventional
Symbol
50 N12
25 N11
12.5 N10
6.3 N9
3.2 N8
1.6 N7
0.8 N6
0.4 N5
0.2 N4
0.1 N3
~
10. Symbol of Lay Interpretation
Parallel to the plane of
projection of the view in which
the symbol is used
Perpendicular to the plane of
projection of the view in which
the symbol is used
Crossed in two slant direction
relative to the plane of
projection of the view in which
the symbol is used
Multidirectional
Approximately circular relative
to the centre of the surface to
which the symbol is applied
Approximately radial relative to
the centre of the surface to
which the symbol is applied
11.
12. The factors affecting surface roughness
• Type of coolant used
• Cutting parameters such as feed, speed and depth of
cut
• Type of machining
• Rigidity of the system consisting of machine tool,
fixture, cutting tool and work
• Vibrations
• Material of tool and work piece.
13. Orders of Geometrical irregularities
Any material being machined by conventional machining
process cannot be finished perfectly. The surface generated
will have some irregularities and these geometrical
irregularities could be classified as follows:
• First Order: It includes the irregularities developed due to the
inaccuracies in the machine tool such as lack of straightness of
guide ways, on which tool post is moving.
• Second Order: It includes tile irregularities developed due to
the vibrations and rigidity of machine tools.
• Third Order: It includes the irregularities due to the cutting
parameters such as cutting speed, feed and depth of cut.
• Fourth Order: It includes the irregularities developed due to
the rupture of the material during the separation of the chip
from the already finished surface of the work piece.
14. Surface Roughness
Average of vertical deviations from nominal
surface over a specified length surface.
Sampling Length: It is the length of Profile necessary for the
evaluation of irregularities to be taken in account. Also called as
“Cut-off” length.
It is measured Parallel to direction of profile.
15. Mean line of the Profile: It is the line that divides the effective
profile such that, within sampling length the sum of squares of
vertical ordinates (y1,y2,...) between effective profile points & mean
line is minimum or Zero.
Center line of the Profile: It is the line that divides the effective
profile such that, the area contained by the profile above & below
the line are equal.
16. 1. Centre line Average (C.L.A. Method)
•Surface roughness is
measured as the average
deviation from nominal
surface.
•It is defined as average value
of the ordinates from mean
line, regardless of the
arithmetic sign of ordinates.
=
1
2
Caln
of CLA Value by
equation 2 is carried out
by Planimeter .
Analysis of Surface Traces(surface finish)
17. 2. Root Mean Square (R.M.S. Method)
•Surface roughness is
measured as the average
deviation from nominal
surface.
•It is defined as square root of
arithmetic mean of values of
square of ordinates of the
surfaces measured from mean
line.
18. 3. Ten Point Height Method
In this method avg.
difference between five
height peaks & five lowest
valleys of surface texture
within sampling length
measured from a line parallel
to mean line & not crossing
the profile.
It is simple method to measure total depth of surface irregularities, but it does
not give sufficient information about surface, as no account is taken of
frequency of the irregularities.
If R = Ten point height of irregularities then,
19. Measurement of Surface finish
1) Stylus Probe Instrument (Contact type)
• These methods enable to determine a numerical value
of the surface finish of any surface.
•Skid or shoe which is drawn slowly over the surface
either by hand or by motor drive. The skid when moved
over the surface follows its general contours and
provides a datum for the measurements.
20. • A stylus or probe which moves over the surface with
the skid. The stylus should be cone shaped with a
spherical tip. This records the micro-geometrical form
of the surface. It moves vertically up and down
relative to skid movement due to roughness of the
surface.
• As the stylus tracks the surface peaks and valleys, its
vertical motion is converted to a time varying
electrical signal that represent surface profile.
• Generally it is desired that if the skid is moving up
then the stylus must also be moving up.
21.
22. 2) Profilometer (Contact type) •It is indicating & recording instrument
used to measure roughness in microns.
•It consist of two units Tracer &
amplifier.
•Tracer is finely pointed stylus mounted
in pick-up unit which consist of
induction coil located in field of
Permanent magnet.
•When tracer(stylus) is moved across the
surface to be tested, it is displaced
vertically up & down due to surface
irregularities.
•This causes induction coil to move in
the field of Permanent magnet & induces
the voltage.
•The induced voltage is amplified &
recorded.
Pick-up unit
23. 2) Tomlinson Surface Meter (Contact type)
Fig.1
Fig.2
This instrument was designed by Dr. Tomlinson.
24. Construction:- 1) The diamond stylus on the surface finish recorder
is held by spring pressure against the surface of a lapped steel
cylinder.
2) The stylus is also attached to the body of the instrument by a leaf
spring and its height is adjustable to enable the diamond to be
positioned conveniently.
3) The lapped cylinder is supported on one side by the stylus and on
the other side by two fixed rollers as shown in Fig.2.
3) The stylus is restrained from all motions except the vertical one
by the tensions in coil and leaf spring.
4)The tensile forces in these two Springs also keep the lapped steel
cylinder in position between the stylus and a pair of fixed rollers.
5) A light spring steel arm is attached to the horizontal lapped steel
cylinder and it carries at its tip a diamond scriber which bears
against a smoked glass.
25. • Working:-
• 1) Any vertical movement of the stylus caused by the
surface irregularities causes the horizontal lapped
steel cylinder to roll.
• 2) By its rolling, the light arm attached to its end
provides a magnified movement on a smoked glass
plate.
• 3) The smoked glass trace is then, further projected at
x50 or x100 magnification for examination. This
instrument is comparatively cheap one and gives
reliable results.
26. 2) The Taylor-Hobson Talysurf (Contact type)
This instrument also gives the same information as the
previous instrument, but much more rapidly and accurately.
27. Construction:- 1) The measuring head of this instrument consists of
a diamond stylus of about 0.002 mm tip radius and skid or shoe
which is drawn across the surface by means of a motorized driving
unit.
2) The arm carrying stylus forms an armature which pivots about
centre piece of E-shaped stamping.
3) On two legs of (outer pole pieces) the E-shaped stamping there
are coils carrying an a.c. current. These two coils with other two
resistances form an oscillator.
Working:- 1) As the armature is pivoted about the central leg, any
movement of the stylus causes the air gap to vary and thus the
amplitude of the original a.c. current flowing in the coils is
modulated.
2) This is further demodulated so that the current now is directly
proportional to the vertical displacement of the stylus only.
3) The Demodulated output is caused to operate a pen recorder to
produced a permanent record & meter gives numerical assessment
directly.
28. References:
1. Engineering Metrology by M.Mahajan, Dhanpatrai
and Company Pvt. Ltd.
2. Engineering Metrology, Author-R. K. Jain,
Publisher, Khanna
